This invention relates to centering means for holes of intramedullary nails, of the type used in bone surgery.
For the surgical repair and stabilization of fractures of long bones, such as the femur, tibia, humerus and fibula, it has long been known to insert an intramedullary nail of appropriate length into the medullar cavity, and to anchor bone fragments to the nail. Anchorage is achieved by means of bolts or pins which are screwed into the walls of the bone on opposite sides of the medullary cavity, and these bolts or pins pass through transverse holes located in the vicinity of the proximal and distal ends of the intramedullary nail. The anchoring bolts are held in position until confronting edges of the broken bone have completely grown back, and are then extracted to allow the intramedullary nail to be removed. The procedure for inserting the holes requires (a) accurate location of the transverse holes in the intramedullary nail, (b) drilling through the cortical and spongy tissue of the bone to provide passages for the bolts, and (c) inserting bolts in such a way as to secure the nail in a suitable position with respect to the bone fragments.
The most critical part of this procedure is to determine, from outside, the location and center of so-called blind holes in the nail; this must be done with the maximum possible accuracy in order to avoid any misalignment of drilled holes and repeated drilling of the bone.
The known method of centering provides for the use of a drilling mask or frame consisting of a longitudinal template which can be attached to the intramedullary nail at its proximal end and which is provided with holes to house bushes for drilling the bone at positions aligned with the holes in the nail.
The centering of holes toward the proximal end of the intramedullary nail, close to the area of attachment to the frame, is a relatively simple operation which does not give rise to any special problems. On the other hand, the centering of blind holes near the distal end of the intramedullary nail is very much more difficult because of the elasticity of the template, and play in the vicinity of coupling between the frame and the intramedullary nail.
A great many centering devices have been proposed, using orientable drilling bushes or a more rigid drilling template with less play in the frame/nail coupling. However, none of these proposed arrangements have been sufficiently rigid; as a result, these proposed arrangements do not ensure the accuracy which is needed for repeatable precision drilling at the distal end of the nail.
It should also be noted that all the known centering devices mentioned above require an X-ray source, which is used by the surgeon to check the hole location with respect to the outer surface of the patient's limb.
The known techniques are hazardous because they require the patient and healthcare personnel surrounding him to suffer repeated and prolonged exposure to X-rays, which are well known to be harmful beyond certain limits, but which cannot be switched off without compromising the final result of an operation. Also, X-rays cannot be used to accurately establish the axial orientation of a hole with respect to the external surface of the limb because the X-ray image is projected onto a plane and cannot be used for a clear determination of any errors in the inclination of the hole.
U.S. Pat. No. 4,667,664 discloses a centering device for the blind holes of an intramedullary nail, wherein a bar supports an X-ray aiming device at one end and the intramedullary nail at the other end; the device is provided with a stabilizing member comprising an arm which connects a median portion of the bar to an intermediate point, between the proximal and distal ends of the nail. But even this device is not sufficiently stable with respect to the distal end of the nail, and therefore the device does not offer maximum assurance of accuracy and stability in the drilling of bone at a distal location. Also, this device again makes use of X-rays, and is therefore unreliable and unsafe.
In our copending Italian application VR92A000081, filed Sep. 22, 1992, we disclose an improved hole-centering device and technique for locating blind distal-end holes in an installed intramedullary nail, whereby use of X-rays is avoided. The disclosure of said copending application is hereby incorporated by reference. However, it is helpful here to summarize that, in said copending application, a jig is detachably secured to the proximal end of an installed intramedullary nail having a transverse bolt hole near the distal end of the nail. The jig comprises an offsetting arm which so mounts an elongate template that its distal end can be flexibly deflected in a geometric plane substantially parallel to the nail. A guide bore near the distal end of the template is on an axis perpendicular to the geometric plane. When the jig is chucked to the nail, the axis of the guide bore of the template is parallel to the axis of the bolt hole of the nail, and both the guide-bore axis and the bolt-hole axis are at identical distances from the offsetting arm. The distal end of the template is adapted to removably mount a metal-detector or probe establishing a magnetic field about a directional axis, such that the metal-detector can be selectively moved or positioned on one and then on the other side of a central location in which the template is locally effectively parallel to the nail. Template positions noted for equal detector signals on opposite sides of the central location enable determination of the central location and therefore the point at which the template guide bore is in axial alignment with the bolt hole of the nail. As disclosed in said copending application, a fixed reference must be first established to the distal end of the involved bone, for support of the otherwise cantilevered distal end of the flexibly deflectable template, with provision for controlled measurable deflection and subsequent clamping of the template to the fixed reference, once the center point has been identified as the half-way location between the two points at which the two equal signals were obtained.